In the polyurethane field, increased interest is being shown in compounds which can be added to the polyurethane polymers to act as fire retardant agents. Particular interest is being shown in compounds which have functional groups reactive with the polyol or polyisocyanate used in preparing the polyurethane so that the fire retardant agent can be copolymerized into the polymer chain. One such group of reactive flame retardants are the polyalkylene glycol phosphites such as those described in U.S. Pat. No. 3,009,939. However, these materials, due to their high OH numbers and crosslinking tendency, are unsuitable for use in flexible urethane foams. In U.S. Pat. Nos. 3,081,331 and 3,142,651, there is disclosed a method of forming polyalkylene glycol polyphosphites having up to 10 phosphite groups in the polymer chain by reacting a trialkyl phosphite with a polypropylene glycol in a molar ratio of 2.1 to 2.5 moles of glycol per mole of phosphite. These materials are also unsuitable for use in flexible urethane foams as a result of their high OH numbers and their tendency to crosslink.
Another attempt at employing reactive flame retardants, described in U.S. Pat. Nos. 3,142,651 and 3,092,651, involves the use of polypropylene glycol poly-hydrogenphosphonates produced by a thermal polymerization. Likewise, polyalkylene glycol hydrogen polyphosphonates have also been produced by transesterifying a secondary hydrogen phosphonate with a polyalkylene glycol according to the procedure outlined in British Patent Nos. 796,446 and 1,011,118. However, many of these materials have relatively high acidity, causing them to react with and thereby deactivate certain catalyst systems generally used in the formation of polyurethane polymers such, for example, as tertiary amine compounds. The first method has the additional drawback of contamination of the product by the alkylene glycol by-product, which contamination is not easily removed.
In order to increase the flame retardancy of some of the above described phosphorus compounds, which have low phosphorus content, the prior art has attempted to incorporate various halogen containing substituents into the above described molecules. Thus U.S. Pat. No. 3,159,605 describes the reaction of halogenated methanes with these compounds. Likewise U.S. Pat. Nos. 3,131,206 and 3,328,493 describe the reaction of chloral with them. However, these materials, like their precursors, have many drawbacks. In particular these products have high OH numbers and low phosphorus content thereby rendering them unsuitable as flame retardants in flexible urethane foams.
In the above cited co-pending U.S. applications, there are disclosed novel polyalkylene glycol vinyl phosphates which are far superior as flame retardant for urethane foams, particularly flexible foam, than any of the above described flame retardants. These vinyl phosphates, however, have one drawback. While they yield foams having excellent flame retardance and physical characteristics, they tend to discolor the center of the bun, thereby rendering the foam objectionable in appearance.
It is an object of the present invention to prepare novel polyalkylene glycol polyphosphorus compounds which are suitable as flame retardants for urethane foams, and in particular flexible urethane foams.
It is another object of the present invention to prepare novel polyalkylene glycol polyphosphorus compounds which, while exhibiting flame retardancy and physical properties comparable with those of the flame retardants described in the above cited co-pending applications, yield foams having good color and appearance throughout. Further advantages of the present invention will become obvious from a reading of the disclosure which follows hereinafter.